Mold for injection molding of cutting tool inserts having air gap of controlled width and method of making such inserts

ABSTRACT

A mold is provided for making a cutting tool insert from a material to be molded, the cutting tool insert having an edge defined by two intersecting surfaces. The mold includes a first surface, a second surface intersecting with the first surface to define an edge, and an air gap between the first surface and the second surface at the edge, the width of the air gap varying in a controlled way along its length. A method for making a cutting tool insert is also disclosed.

BACKGROUND AND SUMMARY

The present invention relates to a mold for injection molding of cutting tool inserts having air gap of controlled width and method of making such inserts having cutting edges with improved properties.

Cutting tool inserts for metal machining made of cemented carbide, cermets, ceramics etc are generally made by powder metallurgical methods mixing by wet milling to a slurry, drying the slurry to a powder with good flow properties, pressing the powder uniaxially to parts of desired shape and sintering. Uniaxial pressing is convenient to use in large scale production. However, it puts some restriction on the shape of the product to be made.

Injection molding is common in the plastics industry, where material containing thermoplastics or thermosetting polymers are heated and forced into a mold cavity with the desired shape. The method is often referred to as Metal Injection Molding (MIM) when used in powder technology. It combines the versatility of plastic injection molding with the strength and integrity of machined, pressed or otherwise manufactured small, complex, metal parts. The process involves mixing fine metal powders with plastic binders which allow the metal to be injected into a mold using standard plastic injection molding machines. Then the binders are removed with solvents and thermal processes and finally the part is sintered. Metal injection molded parts find use in a broad range of applications such as aerospace, automotive etc. The advantage of MIM is the possibility to produce small size parts of complex shape with good tolerances. For that reason MIM is also used in the cemented carbide industry for making inserts of complex shape which cannot be easily made by uniaxial pressing.

Cutting tool inserts must have a sharp and well defined cutting edge. Due to air entrapment in the mold cavity the edge of injection cutting tool inserts will not be sufficiently sharp. For that reason an air gap is used along a parting line or mold insert to facilitate escaping of air. This has previously been addressed by using fixed amounts of air gap or clearance between tool components and thus making it very difficult to avoid flashing and/or rounded edges. It is typically necessary to perform grinding operations on cutting tool inserts formed in this manner to obtain an edge similar to an edge that can be obtained through a conventional powder compaction operation.

It is desirable to provide injection cutting tool inserts formed by MIM with sharp edges while avoiding production of excessive flash or rounded edges. It is also desirable to provide a mold and a method for production of such cutting tool inserts.

In accordance with an aspect of the present invention, a mold for making a cutting tool insert by MIM is provided, the cutting tool insert having an edge defined by two intersecting surfaces. The mold comprises a first surface, a second surface intersecting with the first surface to define an edge, and an air gap between the first surface and the second surface at the edge, the width of the air gap varying in a controlled way along its length.

In accordance with another aspect of the present invention, a method is provided for making a cutting tool insert in a mold, the cutting tool insert having an edge defined by two intersecting surfaces. According to the method, material for forming the cutting tool insert is forced into a mold cavity having a first surface and a second surface, the first surface and the second surface defining an edge. Air is removed from the mold cavity through an air gap between the first surface and the second surface at the edge, the width of the air gap varying in a controlled way along its length.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a metal injection molding machine with a mold.

FIG. 2 is a side, cross-sectional view of a mold according to an aspect of the present invention.

FIG. 3 is a top view of example mold taken at the parting between mold components 10 and 11.

FIG. 4 is a perspective view of a cutting tool insert of the type made with a mold according to an aspect of the present invention.

FIG. 5 is a perspective view of a portion of a mold according to an aspect of the present invention, showing an air gap that varies in width along its length.

FIG. 6 is a side, cross-sectional view of a mold according to another aspect of the present invention.

DETAILED DESCRIPTION

In FIG. 1 a schematic presentation of an injection molding machine and a mold 21 is shown. The material 25 is injected into one or more mold cavities 41. The mold 21 is formed by closing mold components 10, 11 and 12 thus defining the mold cavity 41.

A mold 21 for making a cutting tool insert 23 (FIG. 4) from a material 25 to be molded is seen in FIGS. 1 and 2 in more detail. As seen in FIG. 4, the cutting tool insert 23 has an edge 27 defined by two intersecting surfaces 29 and 31. The mold 21 comprises a first surface 33 and a second surface 35 intersecting with the first surface to define an edge 37.

An air gap 39 is provided between the first surface 33 and the second surface 35 at the edge 37. As seen in FIG. 2, during injection of material 25 to be molded, air ahead of the flow front 43 of the material escapes the mold cavity 41 through the air gap and escapes to an air channel 45 outside of the mold cavity.

The width of the air gap 39 varies in width in a controlled way along its length. The minimum width of the air gap 39 is 0-5 um, and the maximum width of the air gap is up to 20 um. The width of the air gap 39 will typically vary as a function of factors that affect the ability of the material 25, to be molded, to flow to fill the space proximate the edge 37.

For example, in an injection molding operation as shown in FIGS. 1, 2 and 3 the width of the air gap 39 is varied as a function of distance from a sprue gate 47. FIG. 5 shows an edge 37 defined by two intersecting surfaces 33 and 35 with an air gap 39 at the edge. The width of the air gap 39 varies along its length from a largest width W1 at a point 48 for forming a nose of a cutting tool insert to a lesser width W2 at a point remote from the nose, but closer to the sprue gate 47.

The width of the air gap 39 will typically be varied as a function of pressure at the edge 37. For example, as the flow front 43 is more remote from the sprue gate 47, pressure at the flow front is ordinarily reduced and it becomes increasingly difficult to force air from the mold cavity through an air gap of the same width as at a point closer to the sprue gate 47 and a larger air gap facilitates escaping of air.

The width of the air gap 39 is varied as a function of the characteristics of the material 25 to be molded. For example, as the material 25 is more remote from the sprue gate 47, its temperature tends to be lower and it also tends to be more viscous. At points where the temperature of the material is lower and/or it is more viscous, it is useful for the air gap to be larger than when the temperature of the material is higher and it is less viscous.

The mold 21 is considered to be particularly useful for forming cutting tool inserts 23 in the form of cutting inserts with cutting edges 27 as seen in FIG. 4. Such inserts are typically formed by molding an insert from organic binders mixed with cemented carbide powder, and subsequently sintering to form the final product. The powder and binder mixture is introduced to the mold cavity 41, in an injection molding process as seen in FIGS. 1 and 2, through a sprue gate 47 after mold components 10, 11 and 12 have been closed relative to each other to form the mold cavity.

While the air gap 39 at the edge, that is used for forming a cutting edge of a cutting insert, is particularly useful for forming a cutting edge requiring little or no subsequent machining after sintering, as seen in FIG. 6, an air gap 49 can also be provided at a parting line 51 that is at least partly defined by a pin 53 of the mold. Similarly, an air gap 55 is formed at a corner 57 that is at least partly defined by a core 59 of the mold. An air gap is provided at any location in a mold cavity at which it might be desirable to evacuate air to facilitate formation of a feature on the cutting tool insert to be molded. The width of such air gaps is varied to facilitate evacuation of air.

While the air gap 39 can have a variety of suitable forms, FIG. 2 shows a form that is presently considered to be particularly useful. The air channel 45 is substantially entirely formed in a first mold component 10, and a second mold component 11 is formed with a shallow recess 65. It is presently believed to be simplest to vary the depth of the shallow recess 65 to vary the width of the air gap. For example, the shallow recess 65 may have a depth of 0-5 μm at a shallowest depth, and a depth of up to 20 μm at a greatest depth. An aspect of the present invention also relates to a method for making a cutting tool insert 23 in a mold 21. The cutting tool insert 23 has an edge 27 defined by two intersecting surfaces 29 and 31. According to the method, material 25 for forming the cutting tool insert 23 is forced into a mold cavity 41 having a first surface 33 and a second surface 35. The first surface 33 and the second surface 35 define an edge 37. Air escapes from the mold cavity 41 through an air gap 39 between the first surface 33 and the second surface 35 at the edge 37. The size of the air gap 39 varies in width in a controlled way.

In an injection molding operation, material 25 to be molded is injected into the mold cavity 41 through a sprue gate 47. The width of the air gap 39 is varied as a function of distance from the sprue gate 47. The width of the air gap 39 is optimized by simulating mold filling so that the width of the air gap depends on factors such as the distance from the sprue gate 47 and the pressure acting on the material along the air gap, so as to optimize air removal but avoid formation of flash at the edge. By using this method it is thereby possible to produce cutting tool inserts by the use of powder injection molding to net shape, requiring essentially no grinding to create a useable cutting edge on the cutting tool insert.

By using simulation of mold filling for the injection molding process it is possible to calculate the time and the pressure which the material 25 acts on the air gap, and also at what temperature it so does. Close to the sprue gate 47 the material 25 is typically warmer, and possesses lower viscosity, and acts on the air gap 39 a longer time, during injection of material 25, and to avoid flashing, the air gap 39 should be small, 0-5 μm. The farther away from the sprue gate 47 the material 25 flows, the temperature is reduced, which increases viscosity, and the pressure acting on the air gap 39 drops. This, in combination with increasing amounts of entrapped air, makes it useful for the air gap 39 to be increased in width, up to 20 gm, until it reaches a maximum width at the last points at which the mold cavity 41 is filled. Other points of air entrapment can also be identified, e.g. pins 53 and cores 59, and the air gap can be modified accordingly.

Key factors for successful air gap modification are knowledge of characteristic properties of the molded material 25, tool design and the mold filling simulation process. Air gap modification is the key to be able to produce net shape injection molded products possessing similar edge properties after sintering as those produced by the standard powder compaction process.

The present invention has been described with reference to cutting tool inserts. It is obvious that it can be applied also to other products where a sharp, well defined, edge is of importance for the properties of the product.

The disclosures in Swedish patent application No. 0801071-2, from which this application claims priority, are incorporated herein by reference. 

1. A mold 21 for making a cutting tool insert from a material to be molded, the cutting tool insert having an edge defined by two intersecting surfaces and, comprising: a first surface, a second surface intersecting with the first surface to define a edge, and an air gap between the first surface and the second surface at the edge, wherein the width of the air gap varies in a controlled way along its length.
 2. The mold as set forth in claim 1, wherein a minimum width of the air gap is 0-5 μm.
 3. The mold as set forth in claim 2, wherein a maximum width of the air gap is up to 0-5 μm.
 4. The mold as set forth in claim 1, wherein a maximum width of the air gap is up to 0-5 μm.
 5. The mold as set forth in claim 1, comprising a sprue gate, wherein the width of the air gap varies as a function of distance from the sprue gate.
 6. The mold as set forth in claim 1, wherein the width of the air gap varies as a function of pressure at the edge.
 7. The mold as set forth in claim 1, wherein the width of the air gap varies as a function of characteristics of the material to be molded.
 8. The mold as set forth in claim 1, wherein the width of the air gap varies as a function of temperature of the material to be molded.
 9. A method for making a cutting tool insert in a mold 21 , the cutting tool insert having an edge defined by two intersecting surfaces and , comprising: injecting material for forming the cutting tool insert into a mold cavity having a first surface and a second surface, the first surface and the second surface defining an edge, and air escaping from the mold cavity through an air gap between the first surface and the second surface at the edge, wherein the width of the air gap varies in a controlled way along its length.
 10. The method as set forth in claim 9, comprising injecting material to be molded into the mold cavity through a sprue gate, wherein the width of the air gap varies as a function of distance from the sprue gate. 